1. Field of the Invention
The present invention relates to a brake control system for automotive vehicles, and specifically to a computer-controlled braking system with a skid control unit (an ABS unit) acting to prevent a wheel lock-up condition during vehicle deceleration and to provide maximum effective braking by virtue of a computer-controlled, regulated wheel-brake cylinder pressure based on a vehicle deceleration value determined by an input informational data signal from a longitudinal acceleration sensor, and especially suitable for four-wheel-drive vehicles.
2. Description of the Prior Art
As is well known, in typical skid control systems (ABS systems), rotational wheel speed sensors are employed at each road wheel to sense a rotational wheel speed Vw of each road wheel or rotational speeds Vw of at least front-left and front-right wheel speeds detected by two wheel speed sensors located at the front-left and front-right wheels, and a mean rear wheel speed detected by a wheel speed sensor mounted on the rear differential. The ABS system arithmetically calculates or estimates a so-called pseudo vehicle speed Vi on the basis of the wheel speeds detected by the wheel speed sensors, and also arithmetically calculates a pressure-reduction threshold value .lambda.1 on the basis of the estimated pseudo vehicle speed Vi. The pressure-reduction threshold value .lambda.1 is defined as a wheel speed value which may provide an optimal braking condition of the vehicle. Commonly, the ABS system functions to reduce a wheel-brake cylinder pressure within a wheel-brake cylinder of a road wheel which is subjected to skid control so as to prevent a wheel lock-up condition, when the wheel speed Vw of the road wheel subjected to the skid control reaches the pressure-reduction threshold value .lambda.1. In order for the ABS system to accurately calculate or estimate the pseudo vehicle speed Vi, the pseudo vehicle speed based on the wheel speeds Vw is generally compensated for by a deceleration rate (simply a deceleration) .DELTA.Vi. There are two typical ways of determining or deriving the deceleration .DELTA.Vi of the vehicle. One way of determining the deceleration .DELTA.Vi is arithmetic calculation of the deceleration based on the wheel-speed difference between front and rear road wheels. The other way is the use of a signal value from a longitudinal acceleration sensor, which will be hereinafter referred to as a "longitudinal G sensor". On four-wheel-drive automotive vehicles, there is a less tendency for a wheel-speed difference between the front and rear road wheels to occur. Thus, an ABS system mounted on the four-wheel-drive vehicle often uses an input information signal from the longitudinal G sensor, in determining the vehicle deceleration .DELTA.Vi. In such techniques that the pseudo vehicle speed Vi is compensated for by using a signal from the longitudinal G sensor, however, there may be an output (Xg) from the longitudinal G sensor during up-hill driving or during down-hill driving, even when the vehicle is running at a constant speed. The erroneous output from the longitudinal G sensor, occurring owing to the up-hill or down-hill driving, lowers the accuracy of arithmetic calculation for the pseudo vehicle speed Vi. To avoid this, Japanese Patent Provisional Publication No. 10-16748 teaches the use of an offset value corresponding to an inclination of a downhill or an uphill. That is, the output from the longitudinal G sensor, representative of a longitudinal acceleration or a longitudinal deceleration, is compensated for by the offset value. In the anti-skid control device disclosed in the Japanese Patent Provisional Publication No. 10-16748, when the output from the longitudinal G sensor varies during skid control (during ABS operation), an arithmetic and logic section of an electronic control unit (ECU) decides the presence or absence of a shifting action between actual road surface conditions (that is, a shifting action from a high friction coefficient road surface, often called a high-.mu. road, to a low friction coefficient road surface, often called a low-.mu. road, and vice versa), and also decides the presence or absence of a change in a gradient or an inclination of the slope (or a road slope rate) on which the vehicle is running. In order to enhance the accuracy of arithmetic calculation for the pseudo vehicle speed Vi, the anti-skid control device of the Japanese Patent Provisional Publication No. 10-16748 further compensates for the offset value only when the logic section determines there is a change in inclination of the slope.